Disclosed are digital offset printing ink compositions and methods for their preparation. Digital offset printing can be used to produce a composite image, which is made up of a number of component images, where each image is formed from a different ink. This is known in normal color printing processes, such as when printing CMYK images. In CMYK color printing, the number of images is four—i.e. a final image is made up of four component images, being formed from C (cyan), M (magenta), Y (yellow) and K (black) inks respectively.
In such a digital offset printing method, the image is transferred to an intermediate transfer module, e.g., a printing plate or rotating drum under the influence of a holding force to ensure that the image is efficiently transferred and held on the intermediate transfer module. In other embodiments chemical forces or other suitable forces may be used, such as the case when a fountain solution is employed. The force is sufficiently strong to allow accurate accumulation of the image on the intermediate transfer module. The force also is strong enough to ensure that the image is completely transferred from the first surface of the drum, leaving it clean to enable subsequent re-use of the first surface immediately for provision of a subsequent component image to be printed. The transfer module can be a rubber blanket, or a silicone surface on a rotating drum.
Conventional digital offset printing methods, such as CMYK color offset printing methods usually transfer one component image at a time onto the intermediate transfer module, e.g. the cyan ink component image is first transferred from the first surface to the intermediate transfer module, before being transferred to a substrate (such as a piece of paper) which will bear the final, composite image, before a magenta ink component image is transferred to the intermediate transfer module and then to the piece of paper and similarly followed by the yellow and black inks separately and in turn. This process is called “four shot” since there are four separate transfers from the intermediate transfer module to the substrate.
In part of other CMYK color digital offset printing processes, there is a process that enables accumulation of all four component inks (C, M, Y and K) on the intermediate transfer module and then to transfer all of the component images at once from the intermediate transfer module to the substrate (i.e. sheet of paper). The images are transferred one at a time onto the intermediate transfer module, where they are accumulated until the component images have been built up—in a superimposed manner. This process is known as “one shot” since all layers of the composite image are transferred at once to the substrate. Accumulating and transferring four superimposed component images at once is a very intensive process and can reduce the lifetime of the intermediate transfer module. Also, the intermediate transfer module and/or the substrate (e.g. sheet of paper) in such intensive processes often need(s) some prior treatment to be able to receive all four inks at once. This treatment can be in the form of applying a coating to the intermediate transfer module or piece of paper. Some of these problems are caused by complications associated with drying of the composite image on the substrate when all four superimposed inks have been transferred simultaneously. Many of these complications can be solved by the use of a UV curable ink that is not a gelling ink.
For example, a digital offset printing process is known that involves the transfer of a pigmented UV Curable ink onto a silicone printing plate that has been partially coated with a release agent, called a fountain solution, e.g. Novec 7500, commercially available from 3M, Minneapolis, Minn. The ink then can be partially cured using UV light and transferred from the plate to the object, which can be made from paper, plastic or metal, as the case may be. The ink on the object then can be exposed again to UV light for final curing of the ink.
In order to meet digital offset printing requirements, the ink typically should possess many desirable physical and chemical properties. The ink must be compatible with materials it is in contact with, including the printing plate, the fountain solution, the paper and the various rollers. For example, a silicone-coated roller may be coated with a fountain solution, the fountain solution then processed to form an image (e.g., by exposure using a photomask, and the like), and the areas in which the image is to be formed are dried or otherwise removed. The ink then adheres to the areas that do not contain the fountain solution, thus requiring the ink to be capable of adhering to the silicone coated drum, but not mixing or otherwise disturbing the fountain solution. The digital offset printing ink also should meet all functional requirements for transfer and curing.
Inks formulated for digital offset printing applications are different in many ways to other inks developed for Xerox printing applications, including pigmented solid Inks and UV gel inks for Jupiter and Mariner Printer Programs. Digital offset inks contain much higher (up to 10 times) pigment loading and therefore have higher viscosity at room temperature and the temperature of application. In formulating such UV curable inks containing much higher pigment loading, problems may arise in adequately mixing in the pigment. For example, high shear typically is required to mix in the pigment, especially as the concentration of pigment increases and the viscosity of the solution increases. The high shearing action may cause some or all of the material to gel. In addition, high shearing also is typically required to reduce the particle size of the pigment to an acceptable range.
Loading UV curable resins with pigments is known in the art. Curable solid ink compositions have been proposed. Low shrinkage radiation curable solid ink compositions are known that can provide the advantages of handling, safety, and print quality usually associated with solid phase change inks while providing additional breakthrough performance-enabling characteristics such as compatibility with commercially available curable monomers, low jetting temperature, low shrinkage upon cooling from the melt and robustness upon curing. Curable solid ink compositions including those containing dyes and commercially resonated pigments added directly to the ink compositions have been proposed. U.S. patent application Ser. No. 12/642,538 of Marcel P. Breton, et al., filed Dec. 18, 2009, entitled “Curable Solid Ink Compositions,” the disclosure of which is incorporated by reference herein in its entirety, describes a radiation curable solid ink composition comprising at least one curable wax that is curable by free radical polymerization; at least one monomer, oligomer, or prepolymer; at least one non-curable wax; at least one free-radical photoinitiator or photoinitiating moiety; and a colorant; wherein the components form a curable ink composition that is a solid at a first temperature of from about 20 to about 25° C.; and wherein the components form a liquid composition at a second temperature of greater than about 40° C.
U.S. patent application Ser. No. 12/703,817 of Marcel P. Breton, et al., filed Feb. 11, 2010, entitled “Process for Preparing Stable Pigmented Curable Solid Inks,” the disclosure of which is incorporated by reference herein in its entirety, describes a process in which liquid pigment is added to a curable composition, by adding pigment to a solid monomer and dispersant with heating, and then combining the solid pigment concentrate with a curable solid ink base comprising the curable component and initiator. While these UV curable inks are suitable for their intended purpose, they are solid or gel inks, and improved liquid UV curable inks for digital offset printing having high pigment loading and optionally, high viscosity are desired.
Thus, there exists a need for a method of making a UV curable ink composition that does not gel, that contains a high pigment load, and that may have a high viscosity suitable for digital offset printing. The present embodiments are thus directed to methods of making UV curable inks suitable for digital offset printing, and to the inks made by the method.